1. Field of the Invention
The present invention relates to the field of microwave circuits, and more particularly, to a method for producing an integrated microwave assembly.
2. Description of the Prior Art
An integrated microwave assembly (IMA) is an electronic unit functioning at microwave frequencies of above approximately 2 GHz and typically consists of interconnected electrical components inside the IMA and input/output ports outside the IMA to allow connection of the IMA to external assemblies or components.
Electrical components inside the IMA typically consist of transistors, passive circuits, capacitors, chips and the like. These components when placed on the same IMA can interact with one another. This interaction is very undesirable for components which are operated at frequencies above approximately 10 Ghz. To prevent this undesirable interaction between components on the IMA, metallic isolation walls are typically provided on the IMA between electrical components.
One method used to fabricate an IMA with good metallic isolation walls is to machine individual cavities in a non-metallic block of material such as duroid in which to place the electronic chips. Isolation walls are added to the block by creating a plurality of vias around each chip and around each interconnection between chips. These vias are filled with a metallic substance, typically plated copper and/or silver filled epoxy, to create a metallic isolation wall. To obtain good isolation typically requires three isolation walls around each of the electrical components requiring a large amount of real estate on a IMA which limits the number of chips which can populate each IMA.
Another method used to fabricate an IMA with good isolation walls is to machine individual cavities in a metallic substrate, such as aluminum, to various depths. The individual components and chips are placed in the respective cavities such that the portion of the metallic substrate that surrounds each cavity provides electrical isolation between chips.
Although machining cavities on a metallic substrate provides good isolation walls, it is expensive and time consuming since each cavity has to be individually machined inhibiting fabrication of a large number of IMA's by a batch process technique. In addition, the machining cavities results in tolerance limitations which prevent such cavities from being produced with the close tolerances necessary to produce IMA's having repeatable microwave characteristics. Consequently, there is substrate-to-substrate variation of the microwave characteristics of such nominally identical substrates.
In addition to machining tolerances, the electrical chips themselves have fabrication tolerances which results in variations in the location of the chips and the height of the contact pads. If uncertain exists in the location of the electrical components and chips, the length of the interconnection between the chips is uncertain resulting in the inductance added by the interconnection being unknown and variable from substrate to substrate. Manual tuning is typically required to compensate for the unknown inductance created by fabrication and placement tolerances.
Manual tuning typically requires adding an additional tuning circuit to the IMA which takes up valuable space and requires time and expertise to tune the circuit for optimized performance. If an IMA can be fabricated to sufficiently accurate dimensions, known inductance will not be added to the IMA such that manual tuning would not be required.
One method used to more closely control the amount of inductance added to the IMA is to use a laminating process to interconnect the electrical components instead of using wire or ribbon bonding. For this method, a polyimide dielectric film is laminated across the top of the components and the substrate. Vias are laser drilled in the film to expose the contact pads on the electronic components to which it is desired to make contact. A metallization layer which is deposited over the polyimide film layer extends into the via holes and makes electrical contact to the contact pads disposed thereunder. This metallization layer is patterned to form various connections between components on the integrated microwave assembly. However, applying a polyimide dielectric film can cause damage to an IMA circuit since the lamination process involves the application of substantial pressure to the polyimide film which can damage, destroy or modify the performance of the chips and components underneath the polyimide dielectric film.
What is needed therefore, is a method to produce an IMA which provides good isolation between internal components, particularly at frequencies above approximately 10 Ghz, compact isolation walls, and accurate dimension such that after fabrication manual adjustment is not required. The method should not require a laminating process guarding against damage to the IMA. In addition, the method should provide for batch processing to lower the cost of each Integrated Microwave Assembly.